This invention generally relates to electronic systems and in particular it relates to low-pass filters.
The prior art simple low-pass filter shown in FIG. 1 includes transistors 20, 22, and 24; current source 26; capacitor C1; input node 30; and output node 32. The limited high frequency attenuation of the prior art low-pass filter shown in FIG. 1 is due to signal feed forward through the gate-to-drain capacitance (C3) of transistor 24. The maximum attenuation is C3/C1. Since transistors 22 and 24 are not totally off, the feed forward capacitance is usually much higher than C3. Unlike a true R-C filter, which has no zero, the simple configuration shown in FIG. 1 has a zero at a certain frequency.
Generally, and in one form of the invention, the low-pass filter circuit includes: a first compound transistor device coupled between an input node and an output node; a first transistor coupled to the input node, a gate of the first transistor is coupled to a drain of the first transistor; a second compound transistor device coupled between a gate of the first compound transistor device and the gate of the first transistor; a second transistor coupled to the first transistor and having a gate coupled to a gate of the second compound transistor device, the gate of the second transistor is coupled to a drain of the second transistor; a current source coupled to the drain of the second transistor; a first capacitor coupled to the output node; and a second capacitor coupled to the gate of the first compound transistor device.